Hooo-Hooo Hooo-Hooo Vol. 13 Issue 01 - Page 12

WildLife Group of the SAVA Botulism in Wild Birds Dr R D Last – BVSc; M.Med.Vet (Pathology) Avian botulism is a neurological disease of birds characterized by flaccid paralysis of voluntary muscles, due to the ingestion of preformed botulinum neurotoxin produced by the bacterium Clostridium botulinum. This condition is considered as the most common cause of die offs in wild bird populations worldwide. waterfowl, shorebirds, fish eating birds, birds that feed on maggots and avians which frequent landfill sites are considered at highest risk of developing botulism. Botulism in wild raptors is usually associated with improper disposal of poultry carcasses. Vultures appear to be resistant to the botulism toxin. Clostridium botulinum is a soil living bacterium that can produce environmentally resistance spores. These spores are highly resistant to desiccation and extreme temperatures, and themselves are harmless, until the correct environmental factors and anaerobic conditions stimulate them to germinate and undergo vegetative growth. Under certain conditions (increased temperature, high levels of organic material in sediment, low water levels, high water pH, low water oxygen content / anaerobic conditions and increased water salinity) spores germinate and grow producing large quantities of toxin. Raw sewage and rotting vegetation are also suitable substrates for toxin production. This bacterium is particularly common in wetland environments, but also grows well in decomposing carcasses. Toxins can also accumulate in fish as well as invertebrate hosts such as aquatic insects, molluscs and crustacea, as well as maggots feeding on decomposed carcasses. Disease transmission is thought to occur through a variety of mechanisms including There are seven distinct types of neurotoxins produced by Clostridium botulinum (A-G), with most isolates producing only one type of neurotoxin, although isolates that produce multiple toxins do occur. Avian botulism is well described in wild birds being most frequently reported in waterfowl (ducks, geese, swans), Ibis, egrets and pelicans. Most wild avian mortalities, particularly in waterfowl, are associated with type C toxin or the type C/D mosaic, while type E toxin has been associated with mortalities among fish eating birds. Filter feeding 12 • Carcass-maggot cycle where dead fish and birds washing up on beaches of ocean wetlands, lakes and ponds can become sources for C. botulinum growth. Shorebirds and maggot eating birds may then ingest the toxins as they feed on maggots and carrion beetles in these decomposing carcasses. Thousands of toxic maggots can be produced from a single carcass. Ingestion of just two to four toxic maggots can kill a bird and further perpetuate the cycle. • Some mussel populations are believed to create favorable habitat for C. botulinum and accumulate the toxin with no ill effects. These mussels then facilitate the transport of the toxin up the food chain as they are consumed by fish which are in turn consumed by fish-eating water birds. • Prolific growth of algae (blooms) is also believed to contribute to avian botulism outbreaks through the creation of decaying mats of sloughed algae which may lead to low water oxygen levels and a rich growth nutrient medium. This creates an ideal environment for the vegetative state of C. botulinum and resultant toxin production.